DESCRIPTION (applicant's description): The regulation of leukocyte recruitment into inflammatory sites occurs at the level of leukocyte recognition of endothelium. This crucial event is mediated by interactions between the selectin family of adhesion molecules and their leukocyte and endothelial expressed ligands. This proposal is directed at understanding the cellular and molecular basis of adhesion mediated by P-selectin Glycoprotein Ligand-1 (PSGL-l). This adhesion molecule is expressed on all classes of leukocytes and mediates adhesion primarily to endothelial or platelet P-selectin, but also mediates interactions with leukocyte L-selectin. Thus, PSGL- 1 is essential for leukocyte recognition of other leukocytes, platelets, and endothelium, and plays a major role in the recruitment of leukocytes to endothelium and amplification of the inflammatory response. Many features of the PSGL-1 ectodomain required for these interactions have been described, and we have recently shown that interactions between the PSGL-l cytoplasmic domain and the actin cytoskeleton via the linker protein moesin, are essential for binding to P-selectin. PSGL- 1 also serves as the entry receptor for the pathogen responsible for Human Granulocytic Ehrlichiosis (HGE). HOE is a recently described tick-borne infection of humans which infects and proliferates within circulating neutrophils. HGE bacterium bind to an area of PSGL-1 identical to or overlapping the P-selectin binding region, but it is not known what features of this binding site are required for bacterial binding and subsequent entry. In this proposal, we hope to further understand the structural requirements of PSGL-1 interactions with P-selectin and HGE by: 1) identifying residues within the cytoplasmic domain of PSGL- 1 responsible for cytoskeletal attachment and cell adhesion; 2) determining the functional importance of interactions between moesin and the cytoplasmic domain of PSGL- 1; and 3) defining the biochemical features of PSGL-1 required for HOE bacterium binding and entry. These studies should greatly enhance our understanding of the essential components of PSGL- 1 required for interactions with both selectins and an intracellular parasite. The combined information gained from exploring these specific alms will add significantly to our understanding of how PSGL- 1 functions, and should identify new clinical targets for intervention in acute and chronic inflammatory disorders, prevention and/or treatment of HGE, and treatment of other diseases involving the immune system.